TSC2003 datasheet(12/30 Pages) TI2

12 / 30 page

TSC2003

SBAS162G

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REFERENCE MODE

There is a critical item regarding the reference when making

measurements while the switch drivers are ON. For this

discussion, it is useful to consider the basic operation of the

TSC2003 (see Figure 1). This particular application shows

the device being used to digitize a resistive touch screen. A

measurement of the current Y position of the pointing device

is made by connecting the X+ input to the A/D converter,

turning on the Y+ and Y– drivers, and digitizing the voltage

on X+, as shown in Figure 4. For this measurement, the

resistance in the X+ lead does not affect the conversion; it

does, however, affect the settling time, but the resistance is

usually small enough that this is not a concern. However,

since the resistance between Y+ and Y– is fairly low, the

on-resistance of the Y drivers does make a small difference.

Under the situation outlined so far, it would not be possible

to achieve a 0V input or a full-scale input regardless of where

the pointing device is on the touch screen because some

voltage is lost across the internal switches. In addition, the

internal switch resistance is unlikely to track the resistance of

the touch screen, providing an additional source of error.

This situation is remedied, as shown in Figure 5, by using the

differential mode: the +REF and –REF inputs are connected

directly to Y+ and Y–, respectively. This makes the A/D

converter ratiometric. The result of the conversion is always a

percentage of the external reference, regardless of how it

changes in relation to the on-resistance of the internal switches.

REFERENCE INPUT

The voltage difference between +REF and –REF (see

Figure 2) sets the analog input range. The TSC2003 will

operate with a reference in the range of 2V to +VDD. There are

several critical items concerning the reference input and its

wide-voltage range. As the reference voltage is reduced, the

analog voltage weight of each digital output code is also

reduced. This is often referred to as the LSB (Least Significant

Bit) size, and is equal to the reference voltage divided by 4096

(256 if in 8-bit mode). Any Offset or Gain error inherent in the

A/D converter will appear to increase, in terms of LSB size, as

the reference voltage is reduced. For example, if the offset of

a given converter is 2LSBs with a 2.5V reference, it will

typically be 2.5LSBs with a 2V reference. In each case, the

actual offset of the device is the same, 1.22mV. With a lower

reference voltage, more care must be taken to provide a clean

layout including adequate bypassing, a clean (low-noise, low-

ripple) power supply, a low-noise reference (if an external

reference is used), and a low-noise input signal.

The voltage into the VREF input is not buffered, and directly

drives the Capacitor Digital-to-Analog Converter (CDAC)

portion of the TSC2003. Therefore, the input current is very

low, typically < 6

µA.

FIGURE 4. Simplified Diagram of Single-Ended Reference.

FIGURE 5. Simplified Diagram of Differential Reference (Y

Switches Enabled, X+ is Analog Input).

Converter

+IN

+REF

Y+

X+

Y–

GND

–REF

–IN

Converter

+IN

+REF

Y+

REF

X+

Y–

GND

–REF

–IN

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